Stroke, atrial fibrillation, and the management of oral anticoagulation

Atrial fibrillation (AF) is the most common arrhythmia, becomes more prevalent with increasing age and is linked to neurological complications, including most notably ischemic stroke, but also cognitive dysfunction. The recent introduction of direct oral anticoagulants (DOAC) significantly advanced the management of patients with AF. Large-scale randomized controlled trials showed that – for most patients – DOAC are at least as effective as vitamin K antagonists (VKA) in preventing ischemic stroke but have the advantage of a lower risk for intracranial hemorrhage (ICH). However, clinically important patient populations were underrepresented or excluded in these trials, and more refined aspects which are important in clinical practice, including concomitant stroke etiologies or brain pathologies, medication adherence, neuroimaging characteristics or biomarker signatures were not addressed. Therefore, several research gaps and challenges remained for neurologists treating patients with AF, of which we selected 4 aspects to focus on in the following topics that comprise this PhD thesis. The first topic focused on 3 high-risk subgroups of patients with AF treated with oral anticoagulants who were underrepresented or excluded from the large randomized trials. These were (i) stroke patients aged 85 years and older (“the oldest-old”), (ii) severely affected stroke patients dependent on the daily help of others, and (iii) stroke patients with concomitant cerebral small vessel disease. In the first main project of this PhD thesis we examined the performance of DOAC versus VKA in the oldest-old patients with recent stroke and AF in a large pooled analysis across 7 cohort studies. Facing the paucity of randomized evidence, many physicians have been reluctant to use DOAC in these patients. With this project, we provided new evidence that the benefits of DOAC over VKA are preserved in the oldest old with recent stroke, without any signal of a safety concern regarding risk of ICH. In an additional project from our local cohort, we showed that the favourable profile of DOAC over VKA was preserved also among patients with AF and recent stroke who were dependent on the daily help of others, a patient subgroup for which no data existed previously. Finally, we showed in the same cohort that concomitant cerebral small vessel disease in anticoagulated patients with AF and recent stroke was associated with an unfavorable clinical course, but the risk for ischemic stroke remained higher than the risk for ICH, even in the presence of small vessel disease. The latter two projects fall within the scope of this PhD thesis, but do not constitute its main body. Although both manuscripts were published in peer-reviewed journals, for the purpose of this thesis their presentation is restricted to abstracts. Our findings in the first topic of this PhD thesis advance the evidence for the use of anticoagulants in high-risk patient subgroups with AF and recent stroke. The second topic examined ischemic stroke occurring despite anticoagulant therapy in AF patients. With the increasing use of oral anticoagulants, this scenario represents a growing challenge in everyday clinical practice, indicating the need to elucidate the underlying causes and – based on these – the optimal subsequent management strategies. We addressed this issue in the second main project of this PhD thesis in a large retrospective analysis pooling data of prospectively collected patients from 11 stroke centers. We found that the causes of stroke despite anticoagulation in AF patients were heterogeneous, but form three main clusters, all of which were comparably important. These included (i) competing stroke mechanisms other than AF-related cardioembolism, and (ii) insufficient anticoagulation due to prescription errors and nonadherence, suggesting that individualized treatment approaches to address these causes are necessary. Importantly, the third and most common cause was AF-related cardioembolism despite sufficient anticoagulation, indicating the need to develop novel preventive strategies beyond the currently available anticoagulants. Furthermore, in this project we were able to demonstrate that AF patients with stroke despite anticoagulation represent a high-risk patient population, with higher than expected rates of stroke recurrence and other unfavorable outcomes. Finally, we showed also in this population that subsequent treatment with DOAC was associated with better outcomes than VKA treatment. Interestingly, neither any specific switch between DOAC nor antiplatelets as add-on treatment to anticoagulation seemed to confer any benefit, although both approaches are often employed in clinical practice. This study advanced the evidence for the preferential use of DOAC over VKA in AF patients with stroke despite anticoagulation, for whom no data existed so far, while demonstrating the need for more individualized and novel treatment approaches in these high-risk patients. The third topic of this PhD thesis was the adherence of stroke patients to DOAC. Unlike VKA, DOAC require no coagulation monitoring and have short half-lives, which has raised concerns about nonadherence in AF patients treated with DOAC. This is particularly pertinent to patients with stroke, as shown in the second topic of this PhD thesis. In order to examine the medication-taking behaviour and the effect of an adherence-enhancing intervention in patients with recent stroke, we designed, initiated, and undertook the MAAESTRO study. MAAESTRO has been a joint venture with the Pharmaceutical Care Research Group of the University of Basel and has used electronic monitoring as the main method to assess adherence, data on which have been scarce so far. MAAESTRO comprises an initial observational phase and a subsequent randomized controlled interventional phase. MAAESTRO successfully concluded recruitment in July 2021 with reaching the predefined goal of n=130 participants. The observational study phase has now been completed, but as follow-up in the interventional phase is ongoing, the main study results are not part of this PhD thesis. Still, we present the published study protocol and the first results from the observational phase on the patterns of DOAC-taking behaviour, as well as an exploratory analysis on how adherence was impacted by the COVID-19 lockdown as abstracts, as these publications fall within the scope of this PhD thesis, but do not formally constitute its main body. The fourth and final topic of this PhD thesis focused on cognitive dysfunction as a neurological complication of AF. While stroke is a well-known consequence of AF, there is increasing evidence that AF is also linked to cognitive dysfunction independent of ischemic stroke, but the mechanisms underlying this association are unclear. To preserve cognitive function in the growing population of elderly AF patients, a better understanding of these mechanisms is needed. Using data from the multicenter Swiss-AF Cohort Study, in the third main project of this PhD thesis we investigated serum neurofilament light chain (sNfL), a novel blood-based biomarker of neuronal damage, as a tool to explore the mechanisms through which neurological disease occurs in AF. In a cross-sectional analysis, we showed that sNfL is inversely associated with brain volume and cognitive function, thereby demonstrating that it represents a relevant biomarker of brain health in AF patients. Furthermore, we showed that neuronal loss measured by sNfL is associated with age, diabetes mellitus, heart failure, blood pressure and vascular brain lesions, observations which provide mechanistic insights into the occurrence of neurological disease in AF. Finally, in the fourth main project of this PhD thesis, we additionally investigated in this elderly cardiovascular cohort how renal function and body mass index contribute to sNfL levels in order to gain insights into the homeostasis (i.e., clearance and distribution) of this biomarker in the blood compartment. A better understanding of this is necessary towards further establishing this neurological biomarker in cardiovascular and dementia research. We showed that both renal function and body mass index were strongly, inversely associated with sNfL, but only renal function explained a relevant proportion of its variance. With this project we provided evidence for the importance of accounting for renal function in future sNfL-based investigations in elderly cardiovascular populations, in whom chronic kidney disease is highly prevalent

Intravenous thrombolysis for suspected ischemic stroke with seizure at onset

Objective Seizure at onset (SaO) has been considered a relative contraindication for intravenous thrombolysis (IVT) in patients with acute ischemic stroke, although this appraisal is not evidence based. Here, we investigated the prognostic significance of SaO in patients treated with IVT for suspected ischemic stroke. Methods In this multicenter, IVT-registry-based study we assessed the association between SaO and symptomatic intracranial hemorrhage (sICH, European Cooperative Acute Stroke Study II definition), 3-month mortality, and 3-month functional outcome on the modified Rankin Scale (mRS) using unadjusted and adjusted logistic regression, coarsened exact matching, and inverse probability weighted analyses. Results Among 10,074 IVT-treated patients, 146 (1.5%) had SaO. SaO patients had significantly higher National Institutes of Health Stroke Scale score and glucose on admission, and more often female sex, prior stroke, and prior functional dependence than non-SaO patients. In unadjusted analysis, they had generally less favorable outcomes. After controlling for confounders in adjusted, matched, and weighted analyses, all associations between SaO and any of the outcomes disappeared, including sICH (odds ratio [OR](unadjusted) = 1.53 [95% confidence interval (CI) = 0.74-3.14], ORadjusted = 0.52 [95% CI = 0.13-2.16], ORmatched = 0.68 [95% CI = 0.15-3.03], ORweighted = 0.95 [95% CI = 0.39-2.32]), mortality (ORunadjusted = 1.49 [95% CI = 1.00-2.24], ORadjusted = 0.98 [95% CI = 0.5-1.92], ORmatched = 1.13 [95% CI = 0.55-2.33], ORweighted = 1.17 [95% CI = 0.73-1.88]), and functional outcome (mRS >= 3/ordinal mRS: ORunadjusted = 1.33 [95% CI = 0.96-1.84]/1.35 [95% CI = 1.01-1.81], ORadjusted = 0.78 [95% CI = 0.45-1.32]/0.78 [95% CI = 0.52-1.16], ORmatched = 0.75 [95% CI = 0.43-1.32]/0.45 [95% CI = 0.10-2.06], ORweighted = 0.87 [95% CI = 0.57-1.34]/1.00 [95% CI = 0.66-1.52]). These results were consistent regardless of whether patients had an eventual diagnosis of ischemic stroke (89/146) or stroke mimic (57/146 SaO patients). Interpretation SaO was not an independent predictor of poor prognosis. Withholding IVT from patients with assumed ischemic stroke presenting with SaO seems unjustified. ANN NEUROL 2019Peer reviewe

Aetiology, secondary prevention strategies and outcomes of ischaemic stroke despite oral anticoagulant therapy in patients with atrial fibrillation

OBJECTIVE To investigate the aetiology, subsequent preventive strategies and outcomes of stroke despite anticoagulation in patients with atrial fibrillation (AF). METHODS We analysed consecutive patients with AF with an index imaging-proven ischaemic stroke despite vitamin K-antagonist (VKA) or direct oral anticoagulant (DOAC) treatment across 11 stroke centres. We classified stroke aetiology as: (i) competing stroke mechanism other than AF-related cardioembolism; (ii) insufficient anticoagulation (non-adherence or low anticoagulant activity measured with drug-specific assays); or, (iii) AF-related cardioembolism despite sufficient anticoagulation. We investigated subsequent preventive strategies with regard to the primary (composite of recurrent ischaemic stroke, intracranial haemorrhage, death) and secondary endpoint (recurrent ischaemic stroke) within 3 months after index stroke. RESULTS Among 2946 patients (median age 81 years; 48% women; 43% VKA, 57% DOAC), stroke aetiology was competing mechanism in 713 patients (24%), insufficient anticoagulation in 934 (32%) and cardioembolism despite sufficient anticoagulation in 1299 (44%). We found high rates of the primary (27% of patients; completeness 91.6%) and secondary endpoint (4.6%; completeness 88.5%). Only DOAC (vs VKA) treatment after index stroke showed lower odds for both endpoints (primary: adjusted OR (aOR) (95% CI) 0.49 (0.32 to 0.73); secondary: 0.44 (0.24 to 0.80)), but not switching between different DOAC types. Adding antiplatelets showed higher odds for both endpoints (primary: aOR (95% CI) 1.99 (1.25 to 3.15); secondary: 2.66 (1.40 to 5.04)). Only few patients (1%) received left atrial appendage occlusion as additional preventive strategy. CONCLUSIONS Stroke despite anticoagulation comprises heterogeneous aetiologies and cardioembolism despite sufficient anticoagulation is most common. While DOAC were associated with better outcomes than VKA, adding antiplatelets was linked to worse outcomes in these high-risk patients. Our findings indicate that individualised and novel preventive strategies beyond the currently available anticoagulants are needed. TRIAL REGISTRATION NUMBER ISRCTN48292829

A Score for Risk of Thrombolysis-Associated Hemorrhage Including Pretreatment with Statins

Background: Symptomatic intracranial hemorrhage (sICH) after intravenous thrombolysis with recombinant tissue-plasminogen activator (rt-PA) for acute ischemic stroke is associated with a poor functional outcome. We aimed to develop a score assessing risk of sICH including novel putative predictors—namely, pretreatment with statins and severe renal impairment. Methods: We analyzed our local cohort (Berlin) of patients receiving rt-PA for acute ischemic stroke between 2006 and 2016. Outcome was sICH according to ECASS-III criteria. A multiple regression model identified variables associated with sICH and receiver operating characteristics were calculated for the best discriminatory model for sICH. The model was validated in an independent thrombolysis cohort (Basel). Results: sICH occurred in 53 (4.0%) of 1,336 patients in the derivation cohort. Age, baseline National Institutes of Health Stroke Scale, systolic blood pressure on admission, blood glucose on admission, and prior medication with medium- or high-dose statins were associated with sICH and included into the risk of intracranial hemorrhage score. The validation cohort included 983 patients of whom 33 (3.4%) had a sICH. c-Statistics for sICH was 0.72 (95% CI 0.66–0.79) in the derivation cohort and 0.69 (95% CI 0.60–0.77) in the independent validation cohort. Inclusion of severe renal impairment did not improve the score. Conclusion: We developed a simple score with fair discriminating capability to predict rt-PA- related sICH by adding prior statin use to known prognostic factors of sICH. This score may help clinicians to identify patients with higher risk of sICH requiring intensive monitoring

Protocol for MAAESTRO: Electronic Monitoring and Improvement of Adherence to Direct Oral Anticoagulant Treatment—A Randomized Crossover Study of an Educational and Reminder-Based Intervention in Ischemic STROke Patients Under Polypharmacy

Background: Non-adherence to direct oral anticoagulants (DOACs) remains a matter of concern, especially for patients with a recent stroke. However, data on electronically monitored adherence and adherence-improving interventions are scarce.Aims: We aim to use electronic monitoring in DOAC-treated stroke patients to (i) evaluate the effect of an educational, reminder-based adherence-improving intervention, (ii) investigate predictors of non-adherence, (iii) identify reliable self-report measures of adherence, and (iv) explore the association of non-adherence with clinical outcomes.Methods: Single-center, randomized, crossover, open-label study. Adherence to DOACs of polymedicated patients self-administering their medication will be monitored electronically throughout the 12-month-long study following hospitalization for ischemic stroke. After a 6-month observational phase, patients will receive pharmaceutical counseling with feedback on their intake history and be given a multi-compartment pillbox for the subsequent 6-month interventional phase. The pillbox will provide intake reminders either during the first or the last three interventional-phase months. Patients will be randomly allocated to reminders-first or reminders-last.Study outcomes: Primary: non-optimal timing adherence; Secondary: non-optimal taking adherence; timing adherence; taking adherence; self-reported adherence; clinical outcomes including ischemic and hemorrhagic events; patient-reported device usability and satisfaction.Sample size estimates: A sample of 130 patients provides 90% power to show a 20% improvement of the primary adherence outcome with intake reminders.Discussion: MAAESTRO will investigate various aspects of non-adherence and evaluate the effect of an adherence-improving intervention in DOAC-treated patients with a recent stroke using electronic monitoring.Clinical Trial Registration: ClinicalTrials.gov identifier: NCT03344146, Swiss National Clinical Trials Portal SNCTP00000241

Global Cortical Atrophy Is Associated with an Unfavorable Outcome in Stroke Patients on Oral Anticoagulation.

INTRODUCTION Measures of cerebral small vessel disease (cSVD), such as white matter hyperintensities (WMH) and cerebral microbleeds (CMB), are associated with an unfavorable clinical course in stroke patients on oral anticoagulation (OAC) for atrial fibrillation (AF). Here, we investigated whether similar findings can be observed for global cortical atrophy (GCA). METHODS Registry-based prospective observational study of 320 patients treated with OAC following AF stroke. Patients underwent magnetic resonance imaging (MRI) allowing assessment of GCA. Using the simplified visual Pasquier scale, the severity of GCA was categorized as follows: 0: no atrophy, 1: mild atrophy; 2: moderate atrophy, and 3: severe atrophy. Using adjusted logistic and Cox regression analysis, we investigated the association of GCA using a composite outcome measure, comprising: (i) recurrent acute ischemic stroke (IS); (ii) intracranial hemorrhage (ICH); and (iii) death. RESULTS In our time to event analysis after adjusting for potential confounders (i.e., WMH, CMB, age, sex, diabetes, arterial hypertension, coronary heart disease, hyperlipidemia, and antiplatelet use), GCA was associated with an increased risk for the composite outcome in all three degrees of atrophy (grade 1: aHR 3.95, 95% CI 1.34-11.63, p = 0.013; grade 2: aHR 3.89, 95% CI 1.23-12.30, p = 0.021; grade 3: aHR 4.16, 95% CI 1.17-14.84, p = 0.028). CONCLUSION GCA was associated with our composite outcome also after adjusting for other cSVD markers (i.e., CMB, WMH) and age, indicating that GCA may potentially serve as a prognostic marker for stroke patients with atrial fibrillation on oral anticoagulation

Biomarker, Imaging, and Clinical Factors Associated With Overt and Covert Stroke in Patients With Atrial Fibrillation.

BACKGROUND Atrial fibrillation is a major risk factor for stroke and silent brain infarcts. We studied whether a multimodal approach offers additional insights to the CHA2DS2-VASc score in predicting stroke or new brain infarcts on magnetic resonance imaging (MRI) over a 2-year follow-up. METHODS Swiss-AF is a prospective, multicenter cohort study of patients with known atrial fibrillation. We included patients with available brain MRI both at enrollment and 2 years later. The dates of the baseline and follow-up visits ranged from March 2014 to November 2020. The primary outcome was assessed 2 years after baseline and was defined as a composite of clinically identified stroke or any new brain infarct on the 2-year MRI. We compared a multivariable logistic regression model including prespecified clinical, biomarker, and baseline MRI variables to the CHA2DS2-VASc score. RESULTS We included 1232 patients, 89.8% of them taking oral anticoagulants. The primary outcome occurred in 78 patients (6.3%). The following baseline variables were included in the final multivariate model and were significantly associated with the primary outcome: white matter lesion volume in milliliters (adjusted odds ratio [aOR], 1.91 [95% CI, 1.45-2.56]), NT-proBNP (N-terminal pro-B-type natriuretic peptide; aOR, 1.75 [95% CI, 1.20-2.63]), GDF-15 (growth differentiation factor-15; aOR, 1.68 [95% CI, 1.11-2.53]), serum creatinine (aOR, 1.50 [95% CI, 1.02-2.22]), IL (interleukin)-6 (aOR, 1.37 [95% CI, 1.00-1.86]), and hFABP (heart-type fatty acid-binding protein; aOR, 0.48 [95% CI, 0.31-0.73]). Overall performance and discrimination of the new model was superior to that of the CHA2DS2-VASc score (C statistic, 0.82 [95% CI, 0.77-0.87] versus 0.64 [95% CI, 0.58-0.70]). CONCLUSIONS In patients with atrial fibrillation, a model incorporating white matter lesion volume on baseline MRI and selected blood markers yielded new insights on residual stroke risk despite a high proportion of patients on oral anticoagulants. This may be relevant to develop further preventive measures

Renal Function and Body Mass Index Contribute to Serum Neurofilament Light Chain Levels in Elderly Patients With Atrial Fibrillation.

Objective: Serum neurofilament light chain (sNfL) is increasingly used as a neuroaxonal injury biomarker in the elderly. Besides age, little is known about how other physiological factors like renal function and body mass index (BMI) alter its levels. Here, we investigated the association of estimated glomerular filtration rate (eGFR) and BMI with sNfL in a large sample of elderly patients with atrial fibrillation (AF). Methods: This is a cross-sectional analysis from the Swiss-AF Cohort (NCT02105844). We measured sNfL using an ultrasensitive single-molecule array assay. We calculated eGFR using the chronic kidney disease epidemiology collaboration (CKD-EPI) creatinine (eGFRcrea) and creatinine–cystatin C (eGFRcrea–cys) formulas, and BMI from weight and height measurements. We evaluated the role of eGFR and BMI as determinants of sNfL levels using multivariable linear regression and the adjusted R2 (R2adj). Results: Among 2,277 Swiss-AF participants (mean age 73.3 years), eGFRcrea showed an inverse curvilinear association with sNfL after adjustment for age and cardiovascular comorbidities. BMI also showed an independent, inverse linear association with sNfL. The R2adj of models with age, eGFRcrea, and BMI alone was 0.26, 0.35, and 0.02, respectively. A model with age and eGFRcrea combined explained 45% of the sNfL variance. Sensitivity analyses (i) further adjusting for vascular brain lesions (N = 1,402 participants with MRI) and (ii) using eGFRcrea–cys yielded consistent results. Interpretation: In an elderly AF cohort, both renal function and BMI were associated with sNfL, but only renal function explained a substantial proportion of the sNfL variance. This should be taken into account when using sNfL in elderly patients or patients with cardiovascular disease

Impact of type of oral anticoagulants in patients with cerebral microbleeds after atrial fibrillation-related ischemic stroke or TIA: Results of the NOACISP-LONGTERM registry.

Background Cerebral microbleeds (CMBs) may have a differential impact on clinical outcome in stroke patients with atrial fibrillation (AF) treated with different types of oral anticoagulation (OAC). Methods Observational single-center study on AF-stroke-patients treated with OAC. Magnetic-resonance-imaging was performed to assess CMBs. Outcome measures consisted of recurrent ischemic stroke (IS), intracranial hemorrhage (ICH), death, and their combined analysis. Functional disability was assessed by mRS. Using adjusted logistic regression and Cox proportional-hazards models, we assessed the association of the presence of CMBs and OAC type (vitamin K antagonists [VKAs] vs. direct oral anticoagulants [DOACs]) with clinical outcome. Results Of 310 AF-stroke patients treated with OAC [DOACs: n = 234 (75%); VKAs: n = 76 (25%)], CMBs were present in 86 (28%) patients; of these, 66 (77%) received DOACs. In both groups, CMBs were associated with an increased risk for the composite outcome: VKAs: HR 3.654 [1.614; 8.277]; p = 0.002; DOACs: HR 2.230 [1.233; 4.034]; p = 0.008. Patients with CMBs had ~50% higher absolute rates of the composite outcome compared to the overall cohort, with a comparable ratio between treatment groups [VKAs 13/20(65%) vs. DOACs 19/66(29%); p < 0.01]. The VKA-group had a 2-fold higher IS [VKAs:4 (20%) vs. DOACs:6 (9%); p = 0.35] and a 10-fold higher ICH rate [VKAs: 3 (15%) vs. DOACs: 1 (1.5%); p = 0.038]. No significant interaction was observed between type of OAC and presence of CMBs. DOAC-patients showed a significantly better functional outcome (OR 0.40 [0.17; 0.94]; p = 0.04). Conclusions In AF-stroke patients treated with OAC, the presence of CMBs was associated with an unfavorable composite outcome for both VKAs and DOACs, with a higher risk for recurrent IS than for ICH. Strokes were numerically higher under VKAs and increased in the presence of CMBs. Clinical trial registration http://www.clinicaltrials.gov, Unique identifier: NCT03826927

Practical "1-2-3-4-Day" Rule for Starting Direct Oral Anticoagulants After Ischemic Stroke With Atrial Fibrillation: Combined Hospital-Based Cohort Study

BACKGROUND: The "1-3-6-12-day rule" for starting direct oral anticoagulants (DOACs) in patients with nonvalvular atrial fibrillation after acute ischemic stroke or transient ischemic attack recommends timings that may be later than used in clinical practice. We investigated more practical optimal timing of DOAC initiation according to stroke severity. METHODS: The combined data of prospective registries in Japan, Stroke Acute Management with Urgent Risk-factor Assessment and Improvement-nonvalvular atrial fibrillation (September 2011 to March 2014) and RELAXED (February 2014 to April 2016) were used. Patients were divided into transient ischemic attack and 3 stroke subgroups by the National Institutes of Health Stroke Scale score: mild (0-7), moderate (8-15), and severe (≥16). The early treatment group was defined as patients starting DOACs earlier than the median initiation day in each subgroup. Outcomes included a composite of recurrent stroke or systemic embolism, ischemic stroke, and severe bleeding within 90 days. Six European prospective registries were used for validation. RESULTS: In the 1797 derivation cohort patients, DOACs were started at median 2 days after transient ischemic attack and 3, 4, and 5 days after mild, moderate, and severe strokes, respectively. Stroke or systemic embolism was less common in Early Group (n=785)-initiating DOACS within 1, 2, 3, and 4 days, respectively-than Late Group (n=1012) (1.9% versus 3.9%; adjusted hazard ratio, 0.50 [95% CI, 0.27-0.89]), as was ischemic stroke (1.7% versus 3.2%, 0.54 [0.27-0.999]). Major bleeding was similarly common in the 2 groups (0.8% versus 1.0%). On validation, both ischemic stroke (2.4% versus 2.2%) and intracranial hemorrhage (0.2% versus 0.6%) were similarly common in Early (n=547) and Late (n=1483) Groups defined using derivation data. CONCLUSIONS: In Japanese and European populations, early DOAC initiation within 1, 2, 3, or 4 days according to stroke severity seemed to be feasible to decrease the risk of recurrent stroke or systemic embolism and no increase in major bleeding. These findings support ongoing randomized trials to better establish the optimal timing of DOAC initiation